There has been a positive/negative voltage output circuit 100 as shown in, for example, FIG. 22, as a positive/negative output circuit of an electronic device (for example, audio equipment) which requires two power supplies of positive and negative voltages as a power supply. The positive/negative voltage output circuit 100 generates a positive voltage and a negative voltage with the use of two switching controllers 110 and 120.
In concrete, the positive/negative voltage output circuit 100 comprises a boost positive voltage output circuit 100-1 which boosts an input voltage (denoted as “Vin” in the drawing) inputted from the input terminal 101 and outputs a positive voltage from a positive voltage output terminal 100-2, and a negative voltage output circuit 102 which drops the input voltage and outputs a negative voltage from a negative voltage output terminal 103.
The boost positive voltage output circuit 100-1 comprises a boost switching controller 110; a switching element 115; a coil 116; a diode 117 for rectification; a capacitor 118 for smoothing and resistors 119a and 119b for output voltage feedback. The boost voltage switching controller 110 switches ON and OFF the switching element 115 to boost the input voltage.
The boost switching controller 110 comprises an error amplifier 112 which detects an error between a resistance division value of positive voltage outputs yielded by the resistors 119a and 119b for output voltage feedback and a reference voltage 111; a triangular wave oscillator 113 which generates a triangular wave; and a PWM (Pulse Width Modulation) control circuit 114 which controls the switching between ON and OFF of the switching element 115 on the basis of the triangular wave generated by the triangular wave oscillator 113 and an error signal fed from the error amplifier 112.
The PWM control circuit 114 controls a time period during which the switching element 115 is ON (ONDuty) so that the resistance division value by the resistors 119a and 119b for output voltage feedback becomes the same as the reference voltage 111, whereby the boost positive voltage output circuit 100-1 boosts the input voltage and outputs a desired positive voltage (Vo1) from the positive voltage output terminal 102.
On the other hand, the negative voltage output circuit 100-2 is configured almost similarly to the boost positive voltage output circuit 100-1. Namely, the negative voltage output circuit 100-2 comprises a negative voltage switching controller 120; a switching element 125; a coil 126; a diode 127 for rectification; a capacitor 128 for smoothing; and resistors 129a and 129b for output voltage feedback. The negative voltage switching controller 120 switches ON and OFF the switching element 125 to drop the input voltage.
The negative voltage switching controller 120 comprises an error amplifier 122 which detects an error between a resistance division value of a negative voltage output and a reference voltage 129c yielded by the resistors 129a and 129b for output voltage feedback, and a reference voltage 121; a triangular wave oscillator 123 which generates a triangular wave; and a PWM control circuit 124 which controls a switching between ON and OFF of the switching element 125 on the basis of the triangular wave generated by the triangular wave oscillator 123 and an error signal fed from the error amplifier 122.
The PWM control circuit 124 controls a time period during which the switching elements 125 is ON so that the resistance division value yielded by the resistors 129a and 129b for output voltage feedback is the same as a reference voltage 121, whereby the negative voltage output circuit 100-2 drops the input voltage and outputs a desired negative voltage (Vo2) from a negative voltage output terminal 103.
However, the known positive/negative voltage output circuit 100 shown in FIG. 22 requires two sets of expensive switching controllers 110 and 120 and coils 116 and 126, which causes an increase in cost and size of the circuit.
To overcome the above disadvantage, there is proposed a composite power supply unit which outputs both a positive voltage and a negative voltage with one controller and one coil (see Patent Document 1 below).    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2005-168247